The concentrations of individual amino acid in Necturus proximal tubule fluid and cell water will be measured by 14C dansylation and thin layer chromatography. Transluminal electrical potentials will be measured with microelectrodes and Unidirectional luminal fluxes of amino acids will be assessed by an isotope dilution technique under stopped flow microperfusion conditions. These values will be applied for the calculation of the passive permeability of the luminal barrier to basic acidic and neutral amino acids. The previous finding that microperfusion of proximal tubules with simple saline greatly modifies transport of acidic amino acids will be investigated further. Accordingly, the effect of the ionic composition of tubule fluid on unidirectional fluxes of acidic, neutral and basic amino acids will be examined to determine whether electrogenic co-transport with sodium can be documented and whether such manipulations affects the active uptake of and/or permeability to, alpha-amino acids in general.